Week Number Calculator

Find the ISO week number for any date, and vice versa.

What does Week Number Calculator do?

Get the ISO 8601 week number for any date, or find the dates a given week covers. ISO weeks start on Monday and week 1 is the week containing the first Thursday, which is why 1 January is sometimes in week 52 of the previous year.

How to Use Week Number Calculator

1
Pick a dateDefaults to today.
2
Read the week numberWith the ISO week-year.
3
Or work backwardsEnter a week number to see its dates.

Key Features

ISO 8601 week number and ISO week-year for any date
Reverse lookup: the Monday-to-Sunday range for a given week
Day of year and quarter shown alongside
Handles years with 53 weeks correctly
Current week highlighted by default

Frequently Asked Questions

How is the ISO week number defined?

Weeks run Monday to Sunday, and week 1 is the one containing the year’s first Thursday — equivalently, the week containing 4 January. It is the standard used across Europe and in most business software.

Why is 1 January sometimes in week 52 or 53?

Because a week belongs to the year containing most of its days. If 1 January falls on a Friday, Saturday or Sunday, that week has more days in the old year, so it keeps the previous year’s final week number.

Can a year have 53 weeks?

Yes. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. That happens roughly every five to six years.

Is the US week numbering different?

Yes. The common US convention starts weeks on Sunday and counts week 1 as the one containing 1 January, which gives different numbers from ISO. This tool uses the ISO standard.

Runs in your browser

Dates and times are calculated in your browser using its built-in IANA time zone database. Nothing you enter is transmitted.

How the rest of the site handles data